1. Field of the Invention
The present invention relates to a phase measurement system which may be advantageously used, for example, in the measurement of a three-dimensional shape for measuring the shape of a sample by projecting pattern light on the sample and by processing image information obtained by capturing its optical image.
2. Description of the Related Art
As a noncontact method for the measurement of an object's three-dimensional shape, a method called the grating projection method which analyzes the phase distribution obtained from the grating pattern projected on the object is well known in the prior art. The basic components of a measurement apparatus implementing this method are a three-dimensional object (sample), a projector, and a camera. In this arrangement, the camera is disposed in a horizontal plane containing the approximate centers of the projector and the sample, and is placed at a position spaced apart from the projector.
The most important process in the grating projection method is the process of obtaining the phase of an optical image of the sample from the sample itself or from the pattern displayed or projected on the sample. For example, suppose that a periodic grating pattern is projected from the projector onto the sample. Then, an image (projection image) of the sample on which the grating pattern is projected is captured by the camera. Here, images are taken successively by shifting the phase of the grating pattern in small increments. Then, a phase image is obtained by applying a Fourier transform to the thus obtained projection images. This method is called the Fourier transform phase shift method.
However, since a periodic function such as the grating pattern is Fourier transformed, the obtained phase takes any one value within the range of 2π (0 to 2π, −π to π, etc.). Then, the phase discontinuously changes by 2π, and phase aliasing thus occurs. Accordingly, since, in this condition, the depth cannot be determined uniquely, the phase must be corrected by using a step function synchronized to the phase aliasing. That is, processing must be applied so that the phase changes continuously and monotonically without exhibiting phase aliasing. This processing is called phase coupling.
Phase coupling is disclosed in Japanese Unexamined Patent Publication No. 2001-108422. The phase coupling disclosed in this document is performed by utilizing the property that the combined pitch given by the linear sum of patterns having different pitches (each corresponding to the wavelength of a periodic function) is the least common multiple of the individual pitches. For example, the combined pitch of a pattern whose pitch is 2 (wavelength is twice the unit length) and a pattern whose pitch is 3 (wavelength is three times the unit length) is 6 (wavelength is six times the unit length) which is the least common multiple of 2 and 3. This method accomplishes the phase coupling without explicitly generating a step function.
Phase coupling is also disclosed in U.S. Pat. No. 6,075,605. The phase coupling disclosed in this patent document is performed by explicitly generating a step function by using a space coding pattern method in conjunction with a phase shift method. In this case, using coarse space data obtained by the space coding pattern method, the phases obtained by the phase shift method are coupled to obtain accurate three-dimensional measurement results. Here, the space coding pattern method is a method that obtains the three-dimensional shape of a sample by using space code numbers assigned to the space on which pattern light is projected.